Typically, in CT imaging systems, a rotatable gantry includes an x-ray tube, detector, data acquisition system (DAS), and other components that rotate about a patient that is positioned at the approximate rotational center of the gantry. X-rays emit from the x-ray tube, are attenuated by the patient, and are received at the detector. The detector typically includes a photodiode-scintillator array of pixelated elements that convert the attenuated x-rays into photons within the scintillator, and then to electrical signals within the photodiode. The electrical signals are digitized and then received within the DAS, processed, and the processed signals are transmitted via a slipring (from the rotational side to the stationary side) to a computer or data processor for image reconstruction, where an image is formed.
The gantry typically includes a pre-patient collimator that defines or shapes the x-ray beam emitted from the x-ray tube. X-rays passing through the patient can cause x-ray scatter to occur, which can cause image artifacts. Thus, x-ray detectors typically include an anti-scatter grid (ASG) for collimating x-rays received at the detector.
Imaging data may be obtained using x-rays that are generated at a single polychromatic energy. However, some systems may obtain multi-energy images that provide additional information for generating images.
In order to meet the very tight performance standards and generate high quality and artifact-free CT images, a detector typically provides a response that is linearly related to x-ray intensity. Such performance standards typically include a) stability of the detector over time and temperature, b) sensitivity to focal spot motion, and c) light output over lifetime of the detector, as a few examples. In a third generation CT scanner, the relative behavior of adjacent channels is important and typically has tight specifications from channel to channel in order to avoid ring artifacts. This is commonly referred to as channel-to-channel non-linearity variation or channel-to-local average. Also, the drift of a channel from its state of calibration to its state of imaging (of the patient) can cause image artifacts. This variation is generally interpreted as the variation of one pixel to its neighbor. The sources of variation may be due to different components of the image chain such as collimator plate displacement, diode pixel response, and scintillator pixel damage, as examples. Generally, if certain specifications are not met, the variation can cause ring artifacts, bands, or smudges in the images.
ASGs typically include a plurality of plates that are shared between modules, which may include an end plate that is shared between the two. That is, the ASG may extend over two or more modules. End plates shared may be positioned in a gap formed between edge pixels of each module. In this case and in this example, when the plates get deformed or displaced because of thermal drift or thermal gradient, or by G-loading, gains from the two exposed pixels may induce opposite drift. If the drift occurs between calibration and the image state, then ring artifacts or other types of artifacts can be created.
Thus, there is a need to reduce variation within a CT detector and improve the robustness of an ASG.